The present invention relates generally to wheeled vehicle suspension systems and, in an embodiment described herein, more particularly provides a tag axle suspension system.
Tag axle suspension systems are used to increase the load carrying capabilities of a vehicle, such as a dump truck, flat bed truck or a cement truck. When the increased load carrying capability is not needed, the tag axle is raised, so that wheels at each end of the tag axle no longer contact a road surface.
Often, requirements for tag axles tend to make conventional lift axle suspension systems not generally suitable for use as tag axle suspension systems. For example, since tag axles may be positioned a relatively large distance from the nearest non-lift axle, the tag axles should be raised upward a greater distance to avoid contact with the road surface due to suspension jounce, to avoid contact with debris on the road, and to avoid contact with the road surface due to dips or humps in the road surface.
Some present tag axle suspension systems use actuators to raise and lower the tag axles. The actuators are pistons received in cylinders, typically referred to merely as “cylinders”. Specifically, the cylinder of a tag axle suspension system is generally positioned above the tag axle and is extended to lower the tag axle.
Since the cylinder is extended when the tag axle is lowered, the fluid used to extend the cylinder is compressed when the tag axle is pushed upward, for example, due to irregularities in the road surface. To provide increased compressibility, nitrogen gas may be used in combination with hydraulic fluid, for example, by using a nitrogen filled rod. Generally, tag axle suspension systems use this compression of fluid in the cylinder to impart compliance to the suspension systems, i.e., to enable the suspension systems to traverse irregularities in the road surface. However, compression of fluid in cylinders usually causes early failure of seals on the pistons due to long periods of relatively high frequency displacement of the pistons in the cylinders. In addition, these types of systems are relatively expensive, due mainly to the requirement for special-purpose cylinders.
Typical tag axle suspension systems also do not include any provision for maintaining their caster angles during jounce and rebound of the suspension systems. As a result, steering accuracy and ride quality suffer.
These particular tag axle suspension systems are also very large and heavy for the increased load carrying capacity they provide. They are, therefore, somewhat inefficient for their intended purpose.
Furthermore, typical tag axle suspension systems handle braking and suspension loading in a way that is inconsistent with their purpose of increasing load carrying capacity. That is, they transfer braking and suspension loading to the vehicle frame improperly.
From the foregoing, it can be seen that it would be quite desirable to provide an improved tag axle suspension system.